1. Field of the Invention
The present invention relates generally to antenna systems and, more particularly, to an antenna system which produces circularly polarized electromagnetic radiation from a plurality of planar wire grid antennas.
2. Description of the Prior Art
The polarization of an electromagnetic wave can be defined as the directivity and relative motion in space of its associated electric field vector. A "linearly polarized" wave has an associated field vector which describes a line in space (i.e. a vector having no relative motion), and a "circularly polarized" wave has an associated field vector which describes a circle in space (i.e. the field vector has a relative circular motion). The polarization of an antenna is defiend as the resultant plane of propagation of the radiated electromagnetic wave. Where the antenna is oriented such that the direction of propagation is parallel to the plane of the earth, a "horizontally polarized" antenna is one in which the plane of propagation of the wave is parallel to the plane of the earth, and a "vertically polarized" antenna is one in which the plane of propagation of the wave is perpendicular to the plane of the earth. Circularly polarized waves may be produced by the interaction between linearly polarized waves of horizontal and vertical polarizations. In order to achieve proper circular polarization, the horizontally polarized radiating elements and the vertically polarized radiating elements must be perpendicular with respect to each other, fed in phase quadrature, and must radiate at equal electromagnetic intensities; that is, the linearly polarized waves of horizontal polarization and the linearly polarized waves of vertical polarization must be equal in magnitude, orthogonal in orientation, and in time quadrature with respect to each other. If any of these three conditions are not met, elliptically polarized radiation will be produced.
Antennas which produce circularly polarized electromagnetic radiation are generally well known in the prior art. Examples of such antennas are disclosed in U.S. Pat. Nos. 3,680,142 (Van Atta et al); 3,541,559 (Evans); and 4,083,051 (Woodward). In these references, circular polarization is produced by feeding orthogonal radiating elements in phase quadrature. The antennas disclosed in these references present a major antenna design constraint, because each set of orthogonal radiating elements must be separately fed. This "separate feeding" requirement increases the number of feed lines and associated support circuitry needed to produce circular polarization.
In U.S. Pat. No. 3,290,688 (Kraus) there is disclosed a wire grid antenna which produces substantially linearly polarized radiation. With reference to FIG. 1 (prior art), a grid or subarray 1 of rectangular elements is formed by conductors which form the short sides 2, 2' and the long sides 3, 3' of each rectangle. The columns of rectangles are vertically staggered and interconnected such that the short sides of the rectangles in one column connect with the midpoints 4 of the long sides of the rectangles in the adjacent column. The long sides are of a length of approximately one wavelength at the center frequency of operation, and the short sides are approximately one half wavelength long. The subarray of rectangular elements is fed at only one of two points; namely, either at one of the intersection points 5 of a long and a short side of the rectangles along one edge of the subarray, or at one of the midpoints 6 of the short sides of any of the rectangles. By feeding the subarray at a point 5 or a point 6, the long sides of the rectangular elements act mainly as guiding or transmission line elements, while the short sides of the rectangles act as radiating elements. The long sides of the rectangular elements act as transmission line elements due to the fact that the instantaneous current in each long side 3 is equal and opposite to the instantaneous current of the opposite long side 3', whereby their associated electromagnetic radiations cancel (as shown by the arrows in FIG. 1). Likewise, the short sides act as radiators because the currents in opposite short sides 2, 2' are of the same sense, whereby their associated electromagnetic radiations are additive.
In Kraus, the single feed to the antenna at one of the points 5 is a "high impedance, unbalanced" feed, while the single feed to one of the points 6 is a "low impedance, balanced feed". The impedance of the feed is a function of the current density of the radiating element at the feed point. The "balance" of the feed is an expression relating to the grounding of the feed means. An "unbalanced" feed is produced by connecting the inner conductor of a coaxial cable to a feed point 5 and the outer conductor to the ground plane of the antenna. A "balanced" feed, on the other hand, is produced by connecting the two wires 8a and 8b of the conductor to the portions 2a and 2b, respectively, of a radiating element 2 which is open at the center thereof (i.e. at feed point 6) as shown in the lower portion of FIG. 1. The main advantages presented by the Kraus wire grid antenna are that the rectangular grid arrangement allows a high element density to be achieved, the antenna's radiation pattern can be easily modified by changing various parameters of the conductors, and that the antenna is fed at a single feed point. Specifically, the Kraus array realizes a high radiator element density while minimizing the number of feed lines external to the antenna structure.
Heretofore, the above mentioned specific design advantages presented by the Kraus wire grid antenna have not been available for antennas which produce circularly polarized electromagnetic radiation. In addition, it has not been previously possible to further increase the radiator element density of Kraus.